ZOE Science & Nutrition - Recap: What the science says about intermittent fasting | Gin Stephens

Episode Date: February 18, 2025

Today we’re talking about intermittent fasting. This form of time-restricted eating has been gaining significant popularity, with many people reporting positive effects after incorporating it into their daily routines. However, here at ZOE, we want to know what the science says about intermittent fasting. Is it the real deal? Or just another fad? I’m joined by Tim Spector and Gin Stephens to break down what intermittent fasting is, how it affects our bodies, and whether the scientific evidence supports it. 🥑 Make smarter food choices. Become a member a zoe.com - 10% off with code PODCAST 🌱 Try our new plant based wholefood supplement - Daily30+ *Naturally high in copper which contributes to normal energy yielding metabolism and the normal function of the immune system 📚 Books from our ZOE Scientists: The Food For Life Cookbook by Prof. Tim Spector Food For Life by Prof. Tim Spector Every Body Should Know This by Dr Federica Amati Free resources from ZOE: Live Healthier: Top 10 Tips From ZOE Science & Nutrition  Gut Guide - for a healthier microbiome in weeks Have feedback or a topic you'd like us to cover? Let us know here Listen to the full episode here

Hello and welcome to ZOE Recap, where each week we find the best bits from one of our podcast episodes to help you improve your health. Today we're talking about intermittent fasting. This form of time-restricted eating has been gaining significant popularity, with many people reporting positive effects after incorporating it into their daily routines. However, here at ZOE, we want to know what the science says about intermittent fasting. Is it the real deal or just another fad? I'm joined by Tim Spector and Jim Stephens to break down what intermittent fasting is,

how it affects our bodies, and whether the scientific evidence supports it. I'd like to just start at the very beginning, actually, with you, Jen. Okay. What is intermittent fasting? The word sounds so scary. The word fasting makes you think that you're gonna, Let's just start at the very beginning actually with you, Jen. What is intermittent fasting? The words sound so scary.

The word fasting makes you think that you're going to like 40 days and 40 nights wandering in the desert. But intermittent fasting, the word intermittent is key. You are having periods of fasting and periods of eating, which every single person who is listening or watching already does that. It's just changing the balance of that. You go to bed, you sleep, you wake up in the fasted state.

Probably everyone listening has had fasted blood work before. So our bodies are already fasted every single day. If you live an intermittent fasting lifestyle, the difference is you just extend that period. Instead of most people having this much for your feeding time and this much for fasting, we just switch it. And so you're fasting for a longer period of the day, intentionally. And I know that there have been a lot of different types of intermittent fasting

and quite a lot of evolution, I think, about the science and view about what's best. We just explain high level what those are and then talk about what is in fact, I think today seems to be the area that people are most interested in both scientifically and also in terms of just practically being able to carry out and find that they have good experience with. All right. Well, the whole umbrella, intermittent fasting, is any kind of approach where you might have periods where you're intentionally fasting balanced with eating.

And the most common, or what most people think of when they think of intermittent fasting is also known as time-restricted eating. Or if you're like really doing the research in think of intermittent fasting, is also known as time-restricted eating. Or if you're like really doing the research in the lab with rats, you might call it time-restricted feeding. But those are the daily eating window approach where every day you might eat within an eight-hour period or a six-hour period or whatever eating window works for you. And it doesn't have to be the same day to day. There's a whole other branch of also intermittent fasting under that umbrella that is alternate daily fasting. People in the UK probably

heard of it back in the day under the 5-2 name back 2012. That was a big approach, 5-2, 4-3. But most people who do intermittent fasting as a lifestyle tend to gravitate towards time-restricted eating, the daily eating window approach. That's what I do. And the 5-2 wasn't really a proper fast either because it allowed you to have like 500 calories or something. So I think we're talking about something completely different here and the 5-2 has really gone out of fashion anyway. People realized it wasn't as good as it was cut out to be. So I think people are gravitating towards this way of eating which really, as we said, isn't so much fasting as just changing your eating times.

Tim, I think it's a brilliant time to talk about this brand new study, this world's largest study on intermittent fasting called the Big If Study. Could you tell us firstly about the study, what happened, and then what have we found? Yeah, the Big If, it's not a question mark, it's IF, study of intermittent fasting. And it came out of a big health study we were running in the UK called the ZOE Health Study. And there were 246,000 people that were sort of eligible for this. We told them what it entailed, which was just monitoring what you eat for a week so we actually could document what the normal eating windows were. And then the idea was to eat within a 10-hour eating window, 14 hours of overnight fasting

and 10 hours of eating, which isn't a very strict one, but it is one that has been shown across range of animals and humans to be effective. 37,000 of them completed the study. And how long did they have to do the fasting for? They did for two weeks. 27,000 were highly engaged and continued it for many more weeks after that. Some of them up to about 16 weeks and a proportion was still going. So that, you know, we got people hooked on it. At baseline, they weren't too bad that their eating window was about 11 hours.

So for many people they only shifted it by about one hour. The people that did complete it they lost on average about 1.1 kilo grams, that's just over a couple of pounds in weight. But importantly we saw that people who, you know, reduced that eating window most, changed most. And we got increases in energy levels of 22%, mood improved by 11%, and interesting hunger, which most people thought would have gone up, actually slightly dropped. So hang on, you just want to make sure I got that right. You're saying that for the people

who did this, even though they were eating in a shorter time than before, they were actually less hungry than they were before they restricted their eating. That's what they were reporting. Yes. Counterintuitive. Very counterintuitive. When we looked at the people who weren't doing this consistently, so they were sort of having

one day of doing it, another day not. We found that actually things reversed. Those people were more hungry and they had less energy. So clearly do need some sort of stability to this to get these benefits. And we also saw the people that benefited most had an eating window that was largest at the beginning. Those people that maybe were eating over a 14 hour window, they benefited the

most from this and the people that were, you know, marginally near it when they started. So the other thing we found is that people actually had some gut symptoms that improved. So bloating went down by 11%, which was interesting because, you know, there's been some links between people with gut problems. Maybe some doctors say, well, eat little and often that's good for you. But actually we found the opposite. Snacking also was reduced in people. So nearly everybody said they were doing less snacking, which is pretty obvious in a way, but it's nice to actually document that.

So it's one less snack on average per person. We also saw differences in the early time restricted eaters versus the late restricted eaters and this is something Gin knows a lot about. All the studies suggest that metabolically early is slightly better for you if you say stop eating at 6pm rather than at 9pm and they ate less snacks accordingly. But interestingly, when we looked at our data about who completed the study, more people chose to do the late time restricted eating than the early one. Which means they continue to eat into the evening, but they start late,

so they might start eating at 11 o'clock and go through to 9pm, for example, rather than start at 7pm and go through to 5pm. In the UK, that was a more popular option than the other one, which might be finishing 8 in the morning till 6pm at night. So it was a two thirds to one third split on that, which I think is really interesting because it suggests that what we're looking for here is how do we get people to change their habits in a long-term way that's sustainable so they need to choose one that suits them so there's no point telling everyone you must do this in these particular hours otherwise it's

worthless exactly getting people to choose their own regime seemed to be effective and that that's really what we saw so we had 37,000 people who completed this out of 148,000 who signed up for it. So clearly some people didn't like the idea of it once they were forced. A bit like you, Jonathan, if you'd had the choice, you'd say, well, maybe I'll skip that one. But what we are seeing is about a third of people

who weren't picked for their nutrition interest can do this quite effectively and benefit from it. And I think that's the real message out of it is people listening to this, you know, there's a good chance that you'll be in that group and that, you know, we should all be experimenting ourselves and trying it. Yeah. I think what's really important there is you allowed them to choose when they wanted to do it and they naturally chose what felt right to them. And that's what we find in our community, is most people do end up having the window

that shifts a little later. You wake up in the morning, you're already fasting. You know, think about the history of eating and food and what we call things. You're an appetizer, wakes up the appetite. You know, the first time you eat in the day, that wakes up your appetite. So if you just delay when you open your window,

it's a lot easier to keep fasting than it is to stop for the day and close that window when you're used to snacking. For me, I stop eating usually around 7 p.m. every day. That's when I've had enough. I'm not tempted to eat because I'm not hungry anymore between 7 p.m. and bedtime. Now, if I were told I must have an early eating window

and I must open at eight in the morning and then stop, I would be hungry again before bedtime. And that would not be a very enjoyable lifestyle. Whereas I'm sleeping through the part where I would be the hungriest. I wake up and I'm fine. It's been a bit of a shock to me because I was definitely brought up with this assumption that you have to have breakfast. It's really important before you go and do something else. I think also the other misconception we were brought up with is you've got to have food inside you before you undertake any activity. Otherwise, you know, whether it's walking to school or going for a jog or going on your bike, you've got to have carbs up there, you know, to be ready for it.

And now we know that's not true. Yeah, we did a fascinating podcast looking at this only for men in this stage, and we're looking forward to seeing the data also for women looking at actually fasted exercise or not. And actually this amazing fact that actually doing some fasted exercise could actually be beneficial for your health is one of the things that's made me more and more intrigued by this idea that it's not just crazy to. I do all my exercise fasted. Yeah, so do I.

And I think, you know, actually many people now will get out of bed and the best way to do exercise is to not think about it and just do it. And then, you know, once you've done that, then you might start thinking about food. But it's very different to maybe even 10 years ago when, you know, science was, doctors were telling us, you know, you must have reserves before you can do this activity,

otherwise it's all going to fail. So I think everything's being turned on its head. And this is, this is why, you know, we need to be looking at how these sort of lifestyle changes fit into our culture and what we're doing and our way of working. You sort of gave us this overview of the results and I know that these are the preliminary results. What about this for you is surprising or where something sort of new has been discovered? Well looking at the literature, I mean a lot of the stuff on this has been done in mice and clearly

it works super well in mice but humans are not mice, we don't eat like them, we don't sleep like them and you know if you combine most of the studies to date, they've been done small numbers of usually young men, 600-odd participants in total. It hasn't really been broad. So I think it was the first time we'd done a pragmatic study of people to see how many found it easy, how many found it hard. Because the key part of this is not only does it work

if you do it, but how easy is it to do it long term as a lifestyle, and I think this sort of community science study is doing something very different. It's both assessing the acute science for a few weeks, but also really give us real insights into how easy it is to continue it. And I think the surprising other results that other people haven't looked at are things like energy, mood and hunger.

Because there's been a lot of fixation around this around sort of weight loss and sort of cardiovascular, like heart health and things like that hasn't there because you're very hard to ask a mouse, I guess, what its mood is or like how much energy it feels it has. So these things feel very different from that, right? To say you could actually feel different energy in a couple of weeks seems, you know, as just a regular person, that seems completely different from anything to do with like your heart health

or your weight. Yes. So to me, just looking at these results crudely, I'm seeing minor changes on things like weight. You see small changes in metabolism and insulin and things like this. But the only thing I got from the other literature really that I don't think people have highlighted is that you are getting consistent reduction

in inflammation. Now inflammation is this activation of the immune system, and that's the one thing that stood out of all the studies is on average you're getting reduced inflammation from giving your gut a rest, and you are improving your gut microbes, and this has this effect all across our body, and this is why it's important for energy and mood and our immune system in general. And small amounts of weight loss

are a bit of a side effect for some people. They're not the main reason for doing this modest time restricted eating, because it is very modest. You know, we're only shifting people, a lot of them just by one or two hours in a day. It's not a huge difference compared to these trials, which have often combined this with calorie restriction. Thank you for listening to today's recap episode.

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